Avoid gcc dependency

src/sys.rs

@@ -1,16 +1,22 @@
 #[link(name = "syscalls")]
 extern "C" {
+    #[link_name = "_start_window_draw"]
     pub fn start_window_draw();
 
+    #[link_name = "_end_window_draw"]
     pub fn end_window_draw();
 
-    #[link_name = "exit0"]
+    #[link_name = "_exit"]
     pub fn exit() -> !;
 
+    #[link_name = "_define_window"]
     pub fn define_window(ebx: u32, ecx: u32, edx: u32, edi: u32);
+    #[link_name = "_display_message"]
     pub fn display_message(ebx: u32, ecx: u32, edx: u32, edi: u32, esi: u32);
 
+    #[link_name = "_wait_event"]
     pub fn wait_event() -> u32;
 
+    #[link_name = "_pressed_key"]
     pub fn pressed_key() -> u32;
 }